The present invention relates to driers for bulk material which can find utility in the chemical and food industries.
There is known a drier for bulk material fabricated by Krupp of the Federal Republic of Germany consisting of a hollow cylindrical case installed at an angle to the horizontal axis of its rotation and provided with double walls forming a space passed wherethrough is a heat carrier (steam) which is admitted through a passage in a trunnion while condensate is extracted through another passage in the same trunnion. A passage in the other trunnion of the case serves to evacuate the space inside the case.
Known driers require much metal for their manufacture, yet the rate of their throughput is low, an inadequate heating surface being the explanation. The amount of moisture evaporated therein per square meter of the heating surface is also low due to insufficient mixing of the processed material inside the drier and poor contact of the material with the heating surface.
French Pat. No. 1,593,256 discloses a drier for bulk material which is formed of a hollow cylindrical casing rotating about the horizontal axis and containing tubes which are located along the walls all the way along the circumference thereof and parallel to the longitudinal axis. Each of the tubes is welded at its ends to respective tube plates so that the space inside thereof is connected to the space between double walls of the casing a heat carrier passing therethrough. When the drier is in operation, the heat carrier, e.g., steam, is admitted into the space between the double walls of the drier casing through a passage in a trunnion and thence enters the tubes running lengthwise through the space inside the drier casing. The thermal stresses set up in the drier casing differ in magnitude from those subjected to which are the tubes and this difference triggers failures of welds at the joints between the tubes and drier casing.
Another point is that those tubes which are at the bottom of the drier and in contact with the material processed when the drier is set rotating become filled with condensate and fail to provide for a heat exchange as intensive as the upper tubes filled with steam but lacking contact with the material are capable of. The numerous holes the tube plates are pierced with to receive the ends of the tubes reduce the strength of the drier casing and for keeping this strength within a safe limit an increase in the thickness of drier casing is required, calling for high metal requirements.
The longitudinal arrangement of tubes relative to the drier casing prevents an effective mixing of the material processed therein, fails to provide for the equalization of its temperature at the walls and in the medial zone and is consequently incompatible with a sufficiently high performance of the heating surface per unit area. Longitudinally arranged tubes lift some of the material processed, turn it into dust which is carried away, inflicting losses of the product and blocking the system serving the purpose of evacuating the drier. Finally, the longitudinal arrangement of tubes referred to above makes the drier a bulky apparatus and creates no prospect of expanding the surface of heat exchange by a considerable amount no matter how intricate is its construction.